1. Field of the Invention
The present invention generally relates to instruments which operate on the principal of pulse oximetry and more particularly relates to instruments which noninvasively measure oxygen saturation of arterial blood in vivo.
2. Description of the Prior Art
Light in the visible and near infrared region of the electromagnetic spectrum has been used for the in vivo measurement of oxygen saturation levels of a patient's blood. Lewis et al. in U.S. Pat. No. 5,139,025 and Lewis et al. in International Publication (PCT) Number WO 92/21283 discuss spectrophotometric instruments whereby the oxygen saturation of blood, both venous and arterial combined, is estimated using at least three electromagnetic sensor areas. A disadvantage of such instruments is that the accuracy of the oxygen saturation calculation is limited due to such calculation's sensitivity to varying parameters of the tissue other than blood saturation, for example a change in concentration. Rall, et al, in German Patent No. DE 43 04 693 teaches the use of a plurality of light sensors with a single light detector as the best means for oximetry measurement in the particular shape of the device of the invention, primarily intended for connection to a fetus.
New, Jr. et al. in U.S. Pat. No. 4,700,708, the disclosure of which is incorporated by reference, calculates arterial oxygen saturation by isolating the change in detected light intensities during a cardiac cycle in an attempt to minimize and even eliminate the light scattering and absorption effects of non-arterial blood tissue of a patient. Though this technique, known as pulse oximetry, is effective in eliminating many of the artifacts introduced by bone, skin, muscle, etc. a disadvantage exists in that the signal acquisition and computation circuits must be very robust since the useful part of the signal is the relatively small change in detected intensities, as opposed to the total detected intensity. Another disadvantage is that the calculated oxygen saturation value is influenced by pulsatile signal contributions from many differing tissue layers, including the skin or surface tissue layer. It is often desirable to know the arterial oxygen saturation of a particular tissue layer or range of tissue layers as opposed to knowing only a general average arterial oxygen saturation value for all layers, because the oxygen saturation value of the multiple layers may differ from one another. Some clinical conditions, such as stasis, may continue to provide a pulsatile signal in the absence of flow, particularly near the outer surface.
U.S. Pat. No. 5,188,108 issued to Secker, suggests the use of a plurality of emitters and/or receivers to provide multiple emitter/receiver combination. The emitter/receiver spacing for each combination is selected to provide equivalent optical path lengths between combinations using different wavelengths of emission.